Some lighting apparatuses using LED light sources are comprised of a light transmitting optical member in order to control the light distribution characteristics of light from the LED light source. The use of an optical member generally reduces a light output ratio (the light output ratio refers to the ratio of the total luminous flux emitted by the lighting apparatus to the total luminous flux from the light source). To prevent such reduction, it is preferable to use an optical member with a high transmittance.
Furthermore, this type of lighting apparatus comprises a heat transfer member for receiving heat from the LED light source to emit the heat to the outside of the LED source. For example, the heat transfer member is a main body that contacts a back surface of a substrate with the LED light source mounted thereon. For increased heat radiation efficiency, it is preferable that heat is transferred not only to the heat transfer member, but also to the optical member so that heat is also radiated from a surface of the optical member. In this case, it is preferable that the heat-resistant temperature of the optical member is equivalent to the heat-resistant temperature of the LED light source.
Acrylic, which is used as a general optical member, has a high light transmittance, but is lower than LEDs in heat-resistant temperature and has a small coefficient of heat conductivity. Similarly, general polycarbonate has a high heat-resistant temperature, but has a small coefficient of heat conductivity and is lower than acrylic in transmittance. Transparent ceramics have a high heat-resistant temperature and a large coefficient of heat conductivity, but are lower than acrylic in light transmittance and are very expensive.
In other words, no appropriate optical member is available which has excellent heat resistance and which has a high light transmittance and a large coefficient of heat conductivity. This prevents achievement of a satisfactory light output ratio and exhibition of satisfactory heat radiation performance.
Thus, it is desirable to develop a lighting apparatus which has a high light output ratio and which has excellent heat radiation and heat resistance.